1. Field of the Invention
The invention refers to a process for drying brown coals having a high water content, in which process the coal to be dried is, prior to a treatment with saturated steam, sieved and fine grain is separated and the sieve refuse having a grain size of smaller than 300 mm is treated with saturated steam and dried.
2. Description of the Prior Art
In a known process accordrng to Fleissner, the brown coal is dried in an atmosphere of saturated steam under pressures of 10 to 40 bar and at temperatures of 180.degree. to 250.degree. C. Per each ton of raw coal, there are obtained 300 to 800 l of partially strongly contaminated water. The obtained amount of water is dependent on the water content of the raw coal and on the desired drying degree of the dry coal. It is already known to use such water obtained from the process for preheating the coal by spraying this water onto the coal. The process water used for being sprayed over the coal itself has a content of solid matter of approximately 5 to 40 g/l, said content in solid matter containing substantially extremely fine grain. The proportion in solid matter contains only approximately 10% of grains having a grain size of more than 50 .mu.m and approximately 50% of the proportion in solid matter has a grain size of smaller than 10 .mu.m. Furthermore, this process water contains humic acids in amounts of 40 to 150 mg/l and phenols in amounts of 5 to 30 mg/l, so that subsequent purification of such process water is relatively expensive.
From U.S. Pat. No. 4,395,334 there has already become known a process of the initially mentioned type, in which is connected in series a previous sieving step of the coal to be dried. Such a sieving step, which is performed without heating and without the supply of water, does not allow to substantially separate adhering grains, and the material supplied to the drying step performed with saturated steam results, on account of the remaining proportions of phenols, humic acids or the like, in further burdening the waste water of the process. Such contaminations are extremely unfavourable for a biological purification of the waste water, as is in correspondence with the known proposal, and detract from the purification effect.